Demands on semiconductor memory devices toward larger storage capacity and faster access speed have continued to increase. Semiconductor memory devices may be categorized into volatile memory devices and non-volatile memory devices. Dynamic Random Access Memory (DRAM) is a prominent volatile memory device, allowing for high speed and high capacity data storage. Examples of non-volatile memory devices include ROM (Read-only-Memory), EEPROM (Electrically Erasable Programmable ROM), FeRAM (Ferroelectric RAM), and MRAM (Magnetoresistive RAM).
With regard to FeRAM devices, a ferroelectric material is used to store information. The FeRAM devices may include a 1T-1C (1 Transistor-1 Capacitor) memory cell design, similar in construction to a DRAM memory cell, wherein one capacitor and one access transistor form a memory cell. While the dielectric material of DRAM cell capacitor is a linear dielectric material, the dielectric material of FeRAM cell capacitor includes a ferroelectric dielectric material. The FeRAM devices may include a 1T (1 Transistor) memory cell design, based on a ferroelectric field effect transistor (FeFET). For FeFET memory cell, the gate isolation material includes a ferroelectric dielectric material.
Ferroelectric (FE) materials are electrically polarizable materials that possess at least two polarization states, which polarization states may be switched by the application of an external electric field. Each polarization state of FE materials remains stable even after the removal of the applied electric field for at least some period of time. Due to this stability of polarization states, FE materials have been used for memory applications. One of the polarization states is considered to be a logic “1” and the other state a logic “0.” FE materials have a non-linear relationship between the applied electric field and the apparent stored charge, resulting in a ferroelectric characteristic in the form of a hysteresis loop. Several types of FE memory devices have been reported, such as FeRAM devices, and FeFET for NAND and NOR devices.
Perovskite materials, such as lead zirconate titanate (PZT), have commonly been used as FE materials for the FE memory device applications. However, such conventional FE memory devices often fall short in terms of bit density and scalability because perovskite materials exhibit low remnant polarization (Pr). For FeRAM, the thickness of ferroelectric PZT film must be up to 200 nanometers (nm). Thus, the use of conventional FE materials for the sub 20 nm-FE memory devices has been limited. In addition, conventional FE materials, such as PZT, possess limited compatibility with standard semiconductor processing techniques.
Thin films of silicon doped hafnium oxide (SiHfO2) in orthorhombic phase have been investigated as an FE material for FE memory devices. However, the orthorhombic phase of SiHfO2 is not stable, and certain restrictive processing techniques must be utilized in order to stabilize the orthorhombic phase. For example, a titanium nitride (TiN) top electrode may be formed over the thin film of SiHfO2 material, prior to inducing the crystallization of SiHfO2 material through a high temperature annealing process. By crystallizing SiHfO2 material in the presence of an overlying TiN top electrode cap, the orthorhombic phase of SiHfO2 material is formed and stabilized by the mechanically confining (i.e., capping) effect of TiN top electrode, which mechanically strains the underlying SiHfO2 material. It has been reported that by using such SiHfO2 material as the FE material for an FE memory device, the required thickness of the FE material may be reduced to less than 10 nm.
U.S. Pat. No. 8,304,823, issued Nov. 6, 2012 to Boescke, discloses a method for manufacturing a ferroelectric memory cell. An amorphous oxide layer of Hf, Zr or (Hf, Zr) is formed over a carrier, and then a covering layer is formed on the amorphous oxide layer. Upon heating the amorphous oxide layer up to a temperature above its crystallization temperature in the confinement of covering layer (i.e., mechanical capping), at least part of the amorphous oxide layer alters its crystal state from amorphous to crystalline, resulting in a crystallized oxide layer that is suitable as a FE material for an FE memory cell.